1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus to which an ultrasound probe having a capacitive micro-machined ultrasonic transducer can be connected.
2. Description of the Related Art
Conventionally, a ceramic piezoelectric material PZT (lead zirconate titanate) has been used for piezoelectric elements that convert electrical signals into ultrasound in ultrasound transducers. However, a capacitive micro-machined ultrasonic transducer (hereunder, referred to as “c-MUT element”) that is obtained by processing a silicon semiconductor substrate using silicon micro-machining technology is attracting attention.
A c-MUT element includes a planar first electrode that is provided on a silicon substrate, and a planar second electrode that is provided so as to face the first electrode with a predetermined cavity therebetween.
In an ultrasound diagnostic apparatus that generates an ultrasound diagnostic image using an ultrasound probe having the above described c-MUT element, transmission and reception of ultrasound is performed by applying a driving signal to one of the electrodes of the c-MUT element while applying a bias voltage between the two electrodes to thereby cause a film (a membrane constituting the second electrode) at an upper part of the cavity to vibrate and transmit ultrasound, and detecting a returned echo signal with the film at the upper part of the cavity.
That is, the c-MUT element requires not only a driving signal that is an RF signal for generating ultrasound, but also requires a bias voltage at both a time of transmission and reception. Consequently, the sensitivity of the c-MUT element can be controlled by changing the bias level applied thereto.
However, application of an excessive bias voltage to a c-MUT element causes a short circuit in the c-MUT element, and as a result an overcurrent state is entered which leads to damage of the c-MUT element.
To prevent such kind of damage to a c-MUT element due to application of an excessive bias voltage, technology has been proposed in which overvoltage detection means is provided between the electrodes on the c-MUT element side and a bias power source, and application of a bias voltage is stopped when application of an excessive bias voltage is detected by the overvoltage detection means (for example, see Japanese Patent Application Laid-Open Publication No. 2007-29259).
Further, when the size of a bias voltage applied between the electrodes of a c-MUT element exceeds a predetermined range, a drum-shaped sacrificial layer that is inserted between the electrodes enters a crushed state, i.e. a collapsed state. To prevent excessive output of ultrasound to a subject at the time of such a collapsed state, an ultrasound transducer and an ultrasound diagnostic apparatus have been proposed that have a protection circuit that is configured to detect a DC bias voltage, and if the detected DC bias voltage exceeds a threshold value, to cut off an electrical connection between a DC bias circuit and the ultrasound transducer by means of a switch (for example, see Japanese Patent Application Laid-Open Publication No. 2008-136725).
Furthermore, other examples of the prior art include, for example, as disclosed in International Publication No. WO 2005/120130, an ultrasound probe apparatus that, at a time of transmission, superimposes an RF signal, that is, a driving signal, on a DC bias voltage that is outputted from a DC bias generation circuit to an ultrasound transducer of a detachable ultrasound probe.